Double Charged

I have to lines with MTN. When I called in requesting AD on of 3G of Data To my one line, I was given 3G but charge double for it. I paid R660 instead of R330. Again on my other line I requested Data of 2G but on this one I was triple charged. I have been trying to resolve this for the last 20 days with no success. Why is MTN doing this. I have been with MTN for full 12 years really I think its time I cut my services with MTN. This thing of having to call for every Invoice complaining about over charged is really draining. I have been doing it almost 4 months now.